The present invention relates to diesel particulate filters, and in particular to cordierite wall-flow diesel particulate filters having high filtration efficiency, high strength and low pressure drops for catalyzed and uncatalyzed applications.
Porous, wall-flow ceramic filters have been utilized for the removal of carbonaceous soot particles from the exhaust stream of diesel engines since the early 1980s. A porous ceramic diesel particulate filter (DPF) ideally combines low CTE (for thermal shock resistance), low pressure drop (for engine efficiency and fuel economy), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handling, canning, and vibration in use), and low cost. Cordierite has satisfied these requirements; however, benefit would be gained by further reducing the pressure drop of commercially available cordierite DPFs.
During use, the pressure drop of a ceramic filter increases as diesel particulate material or carbon soot is deposited within the filter. Typically, during the deposition of the first 0.5 to 1.0 gram/liter of soot, the increase in pressure drop is especially rapid. This has been attributed to the penetration of soot into the surface and near-surface pores of the filter walls. The presence of soot within the filter walls lowers the overall permeability of the wall, thereby increasing the resistance of gas flow through the wall and increasing the pressure drop across the filter. Once the surface and near-surface pores are filled with the porous soot deposit, additional soot accumulates on the outer surface of the walls of the inlet channels as a discrete porous soot layer, or “soot cake.” Although the permeability of this soot layer is very low, the rate of increase in pressure drop per unit mass of deposited soot is actually lower during the soot layer formation stage than during the initial stage in which soot penetrates the filter wall. Periodically, the soot must be burned out of the filter, in situ, in order to prevent clogging of the filter and to restore the filter to a low pressure drop state. This process is known as “regeneration” of the filter.
A recent trend in pollution control for diesel engines has been the application of a catalyst system to the DPF in order to promote the conversion of carbon monoxide and unburned hydrocarbons in the exhaust stream to carbon dioxide and water. These catalyst systems are generally comprised of a mixture of a highly dispersed noble metal catalyst supported on a high surface area metal oxide. The catalyst system may be present as a “washcoat” on the outer surfaces of the filter wall, but more typically resides primarily within the pores of the filter walls, where the oxides+metals form a thin coating that lines the surfaces of the pore walls.
In addition to catalyzing gas reactions, the catalysts can serve to lower the temperature at which the accumulated soot undergoes combustion during regeneration or can otherwise facilitate the regeneration of the filter. Regeneration of the filter requires that the temperature of the exhaust stream entering the filter be raised to the point at which the soot begins to burn. Various strategies have been adopted to provide this energy, all of which reduce the fuel efficiency of the vehicle. Thus, by reducing the energy that must be supplied in order to light-off of the soot, the catalyst can provide benefits in fuel efficiency and cost savings over the life of the vehicle.
It has been observed, however, that the application of a catalyst system to a DPF causes an increase in the pressure drop across the clean and soot-loaded filter. This is due to a reduction in the porosity and a change in the pore size distribution and pore connectivity of the filter with the application of the catalyst system. This change in pore microstructure reduces the permeability of the clean wall prior to soot loading, and also reduces the permeability of the soot-containing wall for any given mass of soot. In attempting to offset an increase in pressure drop observed in catalyzed filters it has been suggested to increase the porosity and median pore size. However, this combination of high porosity and large pore size causes lower filtration efficiency and lower mechanical strength, and does not yield an optimized filter. Furthermore, non-uniform distribution of the catalyst can result from broad pore size distributions.
A high-porosity cordierite DPF that provides a lower pressure drop in both the catalyzed and uncatalyzed state relative to filters that have been previously demonstrated, while maintaining high filtration efficiency and sufficient mechanical strength, would be an advancement in the art. The present invention provides such a cordierite DPF and a method of making the same.